This invention relates to ultrasound transmit circuits. In particular, ultrasound transmit circuits for generating bi-polar ultrasound waveforms are provided.
For medical diagnostic ultrasound imaging, high current and high voltage amplifiers generate bi-polar waveforms. A wide bandwidth of operation of the amplifiers is used. However, operation of the transmit amplifiers may generate even order distortion products, i.e., such a components at a second harmonic or subharmonics of the fundamental frequency of the bi-polar waveform. The subharmonics and/or harmonics generate undesirable echoes where an ultrasound system is designed to receive valuable information generated by tissue.
Push-pull amplifiers have been used to reduce even order distortion in transmitted ultrasound waveforms. FIG. 1 shows a push-pull amplifier 100 disclosed in U.S. Pat. No. 3,895,306. The push-pull amplifier 100 includes two class A cascode amplifiers 102 and 104 connected in a push-pull relationship with an output transformer 106. Each cascode amplifier includes two transistors 108, 112 and 110, 114. Two transistors 108, 110 are connected in a common base configuration, and the other two transistors 112, 114 operate as common emitter stages.
A network 116 of resistors 124, 126, 138 and a capacitor 118 provides a feedback loop for the push-pull amplifier 100. The network 116 detects differences in the output of the two cascode amplifiers 102, 104 as a function of the current at the center tap 132 of the transformer 106. Any difference at the center tap 132 generates a voltage at the bases of the transistors 108, 110. The voltage is applied to the bases of the transistors 112 and 114 through the capacitor 118 to equalize the cascode amplifiers transfer function. The network 116 provides negative feedback, and the resistors 124, 126 and 138 establish a DC operating voltage. Two input sources 128, 130 provide a signal of the same amplitude but 180xc2x0 out of phase to the cascode amplifiers 102, 104. An inductor 134 isolates the feedback path for the network 116 from a supply voltage 136.
Harmonic and/or subharmonic distortion produced by the two cascode amplifiers 102 and 104 are substantially identical when the fundamental output of the cascode amplifiers 102, 104 are of the same amplitude. The signals output by the cascode amplifiers 102, 104 are equal and opposite. Any even harmonics generated by the cascode amplifiers 102, 104 are cancelled in the output transformer 106. However, any difference in the fundamental waveforms generates a feedback signal. The feedback signal is in phase with respect to the branch with the lower output amplitude and out of phase for the branch with the higher output amplitude. The feedback signal tends to equalize the output of the two branches of the push-pull amplifier 100.
This push-pull amplifier 100 is a Class A amplifier. Class A amplifiers have high quiescent power dissipation, resulting in low efficiency. Higher efficiency is achieved by Class B amplification. For Class B amplification, each path provides output for alternate time periods. The positive and negative portions of the bi-polar waveform are separated for amplification. Consequently, subharmonic and/or harmonic distortions in a Class B amplifier cannot be cancelled by the feedback signal. To reduce these distortions, the two paths are matched in gain and phase.
A high efficiency linear transmit circuit for ultrasound diagnostic imaging is disclosed in U.S. Pat. No. 6,104,673 and is shown in FIG. 2. The transmit circuit 200 operates over a wide frequency bandwidth. The transmit circuit 200 includes a programmable waveform generator (PWG) 202, two digital-to-analog converters 210, 212, a respective pair of current amplifiers or drivers 214, 216 and an output amplifier 218. The output amplifier includes a pair of transistors 222 and 224, and a transformer 220.
The PWG 202 generates separate unipolar waveforms representing positive and negative portions of the desired bi-polar ultrasound waveform. One unipolar waveform is output on bus 206 to a digital-to-analog converter 212, and the other unipolar waveform is output on bus 208 to digital-to-analog converter 210. A sign bit is output on line 204 to enable operation of the digital-to-analog converters 210 and 212. The two transistors 222 and 224 are connected in a common gate configuration. An external voltage source 226 provides gate biasing. A center tap of the primary winding of the transformer 220 is tied to a high voltage power supply 228. Since the transmit circuit 200 includes two open loop signal paths for respective positive and negative portions of the bi-polar transmit waveform, the components in each path should be closely matched to avoid harmonic and/or subharmonic distortion.
In order to transmit a waveform with a Gaussian envelope (FIG. 4A), the current-output DACs 210 and 212 are intended to produce a pair of signals shown in FIGS. 4B and 4C, respectively. Having ideally matched signal paths, transmit signal, U(t), is combined as the algebraic difference of positive, U+(t), and negative, Uxe2x88x92(t), portions in accordance with:
U(t)=U+(t)xe2x88x92Uxe2x88x92(t)xe2x80x83xe2x80x83(1)
Assume further that there is a gain mismatch between the two signal paths, denoted as xcex4=xcex94G/G. In such a case, a xe2x80x9cdistortedxe2x80x9d transmit signal, UD(t), yields
UD(t)=U(t)+xcex4[U+(t)+Uxe2x88x92(t)]xe2x80x83xe2x80x83(2)
The second term of Equation 2 will produce even order distortion products. For instance, given the waveform with the Gaussian envelope, the resulting spectrum expands as shown in FIG. 4D.
In practice, the purity of a transmitted waveform is estimated with the Linear Response Rejection Ratio (LRRR). The LRRR is defined as the ratio of the energy under matched filters that are centered at fundamental and the second harmonic frequencies. For a Gaussian envelope, the LRRR can be easily computed. The obtained results (FIG. 4E) show that the prior art transmit cell 200 is quite sensitive to the gain mismatch. Using a dual DAC topology has a significant drawback since the level of gain mismatch is twice as much higher. This is particularly meaningful because DACs, even high-resolution DACs, may have gain error up to few % of the full scale.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the preferred embodiments described below include a method and system for generating a bi-polar ultrasound transmit waveform. A digital-to-analog converter with differential outputs is connected to two difference amplifiers by current splitters. The difference amplifiers provide current signals to the push-pull output amplifier for generating a desired bi-polar ultrasound waveform. A resistor connecting between the conventional outputs of two differential amplifiers specifies the voltage-to-current scaling factor for both amplifiers. Employing a single resistor, both positive and negative portions of a waveform are uniformly processed. The current splitters allow the digital-to-analog converter to have a low compliance voltage, such as 0.2 or 0.3 volts for an integrated converter, while the difference amplifiers operate at higher voltages for better signal-to-noise ratio performance.
In a first aspect, an ultrasound transmit circuit for generating a bi-polar waveform is provided. An output of a digital-to-analog converter connects with a first current splitter. An ultrasound transducer operatively connects to receive a signal responsive to the digital-to-analog converter.
In a second aspect, an ultrasound transmit circuit for generating a bi-polar waveform includes an output amplifier having first and second inputs. First and second difference amplifiers have respective first and second supply nodes connected with the first and second inputs, respectively. An ultrasound transducer connects with the output amplifier. First and second current splitters connect with inputs of the first and second difference amplifiers. A digital-to-analog converter has differential outputs. A first one of the differential outputs connects with the first current splitter, and a second one of the differential outputs connects with the second current splitter.
In a third aspect, a method for generating a bi-polar ultrasound waveform with an ultrasound transmit circuit is provided. A current from a digital to analog converter is split. An output amplifier is driven in response to at least part of the split current. The bi-polar ultrasound waveform in response to the driven output amplifier.
Further aspects and advantages of the invention are discussed below in conjunction with the preferred embodiments.